20 Unique Cases of Herbicide Resistance in Wisconsin

20 unique cases (weed species by herbicide site of action) of herbicide resistance have been confirmed in Wisconsin, including 13 weed species with evolved resistance to one or more herbicide sites of action (Figure 1, Table 1).

The first confirmed case of herbicide resistance in Wisconsin was PSII inhibitor resistance in common lambsquarters in 1979.

Since then, ALS-inhibitor resistance has been confirmed in more weed species than other type of herbicide resistance, totaling eight weed species including common ragweed, giant ragweed, Palmer amaranth, and waterhemp.

In comparison, PSII inhibitor resistance has been confirmed in four species, whereas ACCase inhibitor resistance has been confirmed in only two species (giant foxtail and large crabgrass).

The first confirmed case of glyphosate (an EPSP synthase inhibitor) resistance in Wisconsin was a non-rapid response phenotype of giant ragweed in 2011 (Figure 2). Glyphosate resistance has subsequently been confirmed in horseweed, waterhemp, Palmer amaranth, and most recently, common ragweed in 2018.